Recent pronounced advancements of solid state image sensors such as CCD and magnetic, electronic or optical recording medium in the field of electronics enabled easy picture-taking as in conventional silver photography, images can be instantly reproduced on television and stored in various recording mediums. Further, as a result of advances in scanners and image processing apparatuses in the field of printing and plate-making, operations such as editing or correcting with watching a color display and treatments such as magnification-reduction or contrast adjustment, after the original images are read, can be easily conducted. Along with such advances, a technique for obtaining high quality images from an image processing apparatus and a recording medium are desired and various types of outputting systems have been put to practical use, including a thermal transfer system, an ink-jet system and an electrophotography system.
Silver halide photographic materials are superior in representation of highlight and image lasting quality and techniques for applying them to digital image output have been developed. The reason why silver halide photographic materials are superior in representation is that silver halide photographic material, as a print material, can form images with little electronic noise. Accordingly, the difference in density which is perceptible in prints is regarded to be substantially in accord with the density difference perceptible by human eyesight.
Silver halide photographic materials are employed not only in photography but also in printing, in terms of high sensitivity, superior color reproducibility and suitability to rapid access. Specifically, the photographic material are employed in the field of so-called proofs to check the finished state prior to actual printing.
Although silver halide photographic materials are quite superior in representation and are broadly employed, conventional silver halide photographic materials have blue-, green- and red-sensitive silver halide emulsion layers, leading to problems such that light sources usable for recording are limited. To expose the silver halide photographic material to light for the purpose described above, gas lasers such as helium-neon, argon gas and helium cadmium have been employed. However, disadvantages of these lasers are that they are too expensive, large and short life. Although a method using a semiconductor laser and a non-linear optical element is known, there were also problems with this method such that its conversion efficiency was insufficient and light with a wavelength suitable for silver halide photographic materials was not available, so that its practical application was not achieved. However, the use of a semiconductor laser and a silver halide infrared-sensitive photographic material makes it possible to readily obtain images using a low-priced compact exposure apparatus.
However, it is generally known that in cases when a silver halide emulsion is spectrally sensitized with an infrared-sensitizing dye, the dye becomes highly unstable in the emulsion, due to its specific structure, producing problems such as increased fogging in spectral sensitization.
To restrain fogging of emulsions, a technique of varying the pAg or the temperature at the time of chemical sensitization is disclosed in JP-A 58-125612 (herein, the term, JP-A means an examined, published Japanese Patent Application). However, this is a technique in chemical sensitization, not a technique for infrared spectral sensitization. JP-A 5-80445 discloses a technique of spectral sensitization, in which a sensitizing dye is added and then the temperature is raised. Thus, the sensitizing dye is added at a temperature 25 to 55.degree. C. and chemical ripening is conducted at a still higher temperature, thereby producing a silver halide emulsion exhibiting superior linearity in the region of from intermediate to high densities. However, there is disclosed nothing with respect to problems concerning specific fogging of a infrared sensitizing dye. Further, the sensitizing dye was added before starting chemical sensitization.
In view of the foregoing, there is continuous strong desire for a technique for achieving reduced fog and enhanced sensitivity, even when subjecting a silver halide emulsion to infrared spectral sensitization.